how do you create the stretched effect on clothes when you move your rigged body
You can create extra bones for the strap and use the stretch to constraint.
You will need to create at least 2 bones per strap, one that is parented to the head, and is located where the strap is tied to the head (this one is the target) and another that is parented to the neck, that runs along the strap *this is the strap bone). The strap bone should have a stretch to constraint targeting the target bone. 

The strap should be weight painted to the strap bone, the target bone should not deform the mesh. 

You can also make a chain of bones for the strap with a stretch to constraint on each bone. To get this to work you will need to reduce the influence slider to some value less than one on each bone except the last (this can require a bit of fiddling to get right though). I would recommend trying one strap bone first though. 
Alternatively, if you want the strap to curve a little you can also look at bendy bones